The invention will be elucidated with reference to components of an offshore wind turbine. The reference to such a wind turbine does not however imply that the invention is limited thereto, and the device and method could be applied for lifting any other object, such as for instance used for offshore foundation structures, jetties, radar and other towers, and the like.
A vessel in a water mass is subject to movements according to six degrees of freedom. Translational movements comprise heave, sway and surge. Where heave is a vertical movement, sway is the side to side or port to starboard movement and surge is the front to back or bow to stern movement. Rotational movements comprise pitch, roll and yaw. Where pitch is a rotation of a vessel about its lateral (port-starboard) axis, roll is the rotation about its longitudinal (bow-stern) axis, and yaw is the rotation about its vertical axis. Heave, sway, surge, pitch, roll and yaw movements are defined in a coordinate system fixed to the floating vessel and therefore may differ from movements in the vertical and two horizontal directions, defined by a coordinate system fixed to the outside world.
The motions of the water mass are transferred to a vessel that is floating in the water mass. The induced vessel movements, in turn, impose movements and accelerations on objects that are transported by the vessel or manipulated on the vessel. When lifting an object from a deck of a floating vessel with a crane for instance, the relative movement between the floating vessel and the object during the lifting operations may cause the object to re-hit with parts of the vessel, or with another structure on the vessel. This is especially the case when the lifting crane is not provided on the floating vessel itself, but is provided on another floating vessel and/or on a stabilized vessel or other platform, such as a jack-up platform that is separate from the object's vessel. Re-hitting the object with the floating vessel could damage the lifted object, parts of the vessel, both, or other items in the vicinity of the lifted object. For instance, after a re-hit, the object may be subject to uncontrolled swinging movements.
According to the state of the art, the risk for a re-hit of the lifted object with the vessel during lifting may be avoided by providing a system that is configured to actively compensate movements of the object relative to the deck of the vessel. Such a system may for instance compensate two rotational movements (pitch and roll) and one translational (heave) movement of the vessel in order for a platform carrying the object to stay substantially horizontal at a fixed height relative to an outside world reference system while the vessel is moving in pitch, roll, and heave directions underneath. The known active compensation system is operable in conjunction with a fixed crane, typically provided on the deck of a jack-up platform, separate from the floating vessel carrying the objects to be lifted. The active compensation will hold the platform and the object on top of the platform substantially stationary relative to the fixed crane. The legs of the jack-up vessel provide the required stability.
Although the known system may be used in some circumstances, it lacks flexibility. It cannot be used for larger objects. Also, the active compensation has to be operable before, during and after lifting, and the risk for material and personal damage is substantial should the compensation for some reason fail temporarily.